In-game currency isn't exactly hard to acquire in Happy Aquarium, but the game must think it is, cause yesterday, it's released a Lucky Slots Idol
that offers up to one million coins for spinning it once every eight hours. The idol itself will be available until May 23rd 6:00 p.m. PST, which is basically the next three days. That's quite a long time for a Limited Edition item.
As for cost, the Lucky Slots Idol is 179 Pearls or 149 Facebook Credits, which is roughly $15 USD.